The brilliance of lamps in a room noticeably drops as soon as a high power electric iron is switched on and after a short interval , the bulbs regain their original brilliance . Why ?

Ans. The cold resistance of the coil of the iron is much smaller than when it is hot . So when the iron is switched on , there is a large drop of voltage and consequently the bulb do not receive the proper voltage . As the coil gets heated , its resistance increases and so the voltage drop is made up when it is fully heated . This is why after some time the bulbs regain their brilliance .